The present invention relates to the field of welding devices. More particularly, this invention relates to welding devices through which metal welding wire is fed and methods of charging and delivering welding wire to an oppositely charged work piece through such devices.
In general, metal-arc inert gas (MIG) welding is a type of shielded-arc welding that is well known and widely used. Shielded-arc welding is based on the principle of protecting the molten filler metal by covering it with chemically inert gas. A power supply charges the work piece to be welded with an electrical potential or charge of one polarity while a metal wire electrode is dispensed through a torch and charged with an electrical potential of an opposite polarity. Meanwhile, an inert gas is delivered through the torch to the free end of the electrode to cover the electrical welding arc which occurs when the charged electrode is brought close to the oppositely charged work piece. When the arc is initiated, the electrical potential or voltage provides a welding current that melts the wire electrode and results in molten filler metal being deposited on the work piece to form a weld. Various types of electrodes have been used in MIG welding, including steel and aluminum wire.
Many MIG welding systems are manually operated with a trigger on the torch, but recently automated systems have been developed. However, problems associated with delivering the wire electrode through the welding torch have generally hampered the success of automated MIG welding systems, particularly when the wire electrode is aluminum. A conventional MIG welding system has a wire delivery system with a wire drive assembly that pulls the wire from an external reel. The wire drive assembly includes an opposing wheel device having either one set of two rolls or two sets of two rolls grippingly engaging the wire electrode therebetween. The rolls are typically made of steel. Each set of rolls grips the wire with adjustable pressure as the result of compression spring which urge the rolls together. The outer circumference of each roll has a V-shaped annular groove for grinding and gripping the wire. The size of the groove matches the diameter of the wire being pulled. The rolls are geared together and driven by a variable speed motor whose speed is regulated by a wire drive control.
An adapter and a three to fifteen foot torch cable are connected to the wire drive assembly and receive the wire as it is pushed from the rolls. The adapter is typically made of copper or brass. Welding power is input through a lug on the adapter and is routed through the cable. The torch cable also houses various wires, hoses and passageways for "utilities", such as cover gas, power and control, and optional cooling water.
The cable is connected to a welding pull torch that delivers the wire electrode and cover gas to the point of welding at the work piece. A motor drives a set of drive rolls mounted in the housing of the torch to assist the wire drive assembly by pulling the wire through the cable and the torch. Finally, the wire electrode is pushed through a precision bore in a contact tip mounted in the nozzle of the torch. The weld power is transferred to the wire electrode by the contact tip, which is a consumable in the welding process. Copper or copper alloy materials are typically used to provide a contact tip that is an excellent conductor.
The positive terminal of an electrical power supply is connected to the lug on the adapter. Electrical power is also provided for the variable speed motor of the wire drive assembly. Power extends to the motor in the pull torch via the cable. Furthermore, the contact tip is connected to the power supply by a wire through the cable so that the contact tip can transfer welding power to the wire electrode as it exits the torch. The oppositely charged terminal of the power supply is connected to the work piece.
Some properties of aluminum, such as low melting temperatures, low electrical resistance and high thermal emissivity, are advantageous for MIG welding. However, certain other properties of aluminum, such as high malleability, low columnar strength and tendency to naturally produce an abrasive oxide coating (aluminum oxide), lead to difficulties in utilizing aluminum wire electrodes in existing MIG welding devices and processes. Many of these difficulties are less pronounced or almost insignificant if steel electrodes are used. For instance, the aluminum wire electrode abrades the conduit in the cable, the contact tip, and the rolls, whereas a steel wire electrode essentially does not. Steel is more resistant to the deformation and shaving that can result when the wire electrode is engaged by the rolls. Gripping an aluminum wire electrode without deforming it is difficult. Although "bird nesting" or jams of the wire electrode are possible with steel, they are much more likely with aluminum because of its low columnar strength and easily deformable cross section.
There is a need for an improved MIG welding device and process, particularly one better suited for using an aluminum wire electrode. Therefore, an object of the present invention is the provision of a metal-arc inert gas (MIG) welding torch assembly and method that constitute significant improvements over existing weld torch assemblies and methods.
Another object of this invention is the provision of a MIG welding torch assembly that minimizes the mechanical inertia that the wire electrode must overcome to initiate a welding arc.
Another object of this invention is the provision of a MIG welding torch assembly having low electrical inertia to overcome at arc starting.
Another object of this invention is the provision of a MIG welding torch assembly and method that minimizes the contact of the wire electrode with other metal surfaces.
Another object of this invention is the provision of a MIG welding torch assembly having reduced risk of internal electrical sparking.
Another object of this invention is the provision of a MIG welding torch assembly that is well adapted for using a variety of types of metal wire electrodes, including but not limited to steel and aluminum.
Another object of this invention is the provision of a MIG welding torch assembly that is particularly well adapted for using aluminum wire electrodes.
Another object of this invention is the provision of a MIG welding torch assembly capable of being manipulated either manually or automatically.
Another object of this invention is the provision of a MIG welding torch assembly having a unique method and structure for wire acceleration.
Another object of this invention is the provision of a MIG welding torch assembly that is adapted for use with a power supply that generates a constant current.
Another object of this invention is the provision of a MIG welding torch assembly that reliably delivers high quality welds.
Another object of this invention is the provision of a MIG welding torch assembly that is durable in use and economical to manufacture.
This invention contemplates other objects, features and advantages which will become more fully apparent from the following detailed description taken in conjunction with the accompanying drawings.